Synchrotron beam time is requested for a number of crystallographic projects probing aspects of protein-protein and protein-ligand interactions. Factor XIII, a blood-clotting enzyme, and its mutants crystallize in several crystal forms. The large size of the protein (dimer molecular weight of 160,000), the large unit cell sizes, and small crystals require the high flux of synchrotron sources for efficient data collection. The structure analysis of another blood-clotting enzyme, factor XII, and its complex with a small protein inhibitor will make use of synchrotron radiation for routine or MAD data collection. Finally, several streptavidin mutants require synchrotron time for ultra high resolution data collection.